Chronic inflammation of the esophagus due to gastroesophageal reflux disease (GERD) contributes to the development of Barrett's esophagus and esophageal adenocarcinoma, a lethal tumor whose incidence has been rising at an alarming rate. Modern medical therapy of GERD is directed almost exclusively at decreasing gastric acid production with medications such as proton pump inhibitors (PPIs). In GERD patients taking PPIs, weakly acidic gastric juice frequently refluxes into the esophagus. Perhaps this is why PPIs do not completely eliminate GERD symptoms in up to 40% of patients, and why the frequency of esophageal adenocarcinoma continues to rise despite the widespread use of PPIs. New medical treatments are needed to improve GERD symptom control, and to prevent the development of Barrett's esophagus and esophageal adenocarcinoma. In a rat model of reflux esophagitis in which an esophago-duodenostomy is created to induce reflux, we recently studied the early histological events in the development of reflux esophagitis, and found a sequence suggesting that this esophagitis develops through a cytokine-mediated injury rather than through an acid burn. Based on these experiments, we proposed a new concept for the development of reflux esophagitis in which the reflux of gastric juice stimulates esophageal squamous epithelial cells to secrete cytokines that induce proliferative changes and attract inflammatory cells, and it is those inflammatory changes, not the caustic effects of acid, that ultimately damage the esophageal mucosa. Hypoxia, a common feature of chronically inflamed tissues, contributes to inflammation through the regulation of HIFs, a family of transcription factors that allow cells to respond to hypoxic stress. Recently, in a mouse model of colitis caused by a genetically-engineered increase in HIF-2? signaling in colonic epithelial cells, colitis was shown to develop in a pattern virtually identical to that which we observed in our rat model of reflux esophagitis. In addition, we have preliminary data demonstrating that exposure to a weakly acidic bile salt solution activates HIF-2? signaling in esophageal epithelial cells. Therefore, we hypothesize that esophageal secretion of pro-inflammatory cytokines and esophageal epithelial cell proliferation result from reflux-induced activation of HIF-2?. The aims of this study are (1) to elucidate the mechanisms whereby acid and bile salts activate HIF-2?, (2) to determine the role of HIF-2? in acid and bile salt-induced expression of pro-inflammatory cytokines and epithelial cell proliferation in esophageal squamous cells in vitro, and (3) to elaborate the early histological events in the pathogenesis of reflux esophagitis in the esophagus of patients with GERD, and to correlate those events with esophageal expression of HIF-2? and pro-inflammatory cytokines, and with changes in esophageal proliferation. PUBLIC HEALTH RELEVANCE: The relevance to public health is that understanding the early pathophysiology of gastroesophageal reflux disease (GERD) at the cellular and molecular levels is essential if new medical treatments are to be developed to improve GERD symptom control, to eliminate esophageal inflammation, and to prevent the development of Barrett's esophagus and esophageal adenocarcinoma.